Using diode-pumped solid-state lasers to generate deep UV around 200 nm is greatly desirable for photo-refractive surgeries. Such a deep UV laser source is expected to be more compacts, more reliable, and less maintenance in comparison with the excimer lasers, which is currently the dominant laser source for photo-refractive surgeries. More importantly, solid-state laser sources can be operated at a much higher repetition rate and much less energy fluctuation than the excimer lasers. Scanning a deep UV laser beam with high repetition rate enables a variety of ablation shapes on a cornea surface and provides a great flexibility for the refractive surgeries. The improved stability in pulse energy from a solid-state UV laser source ensures accurate and controllable ablation.
To obtain such a diode-pumped solid-state laser source, a pulsed infrared laser beam with suitable laser parameters should be first generated and then efficient wavelength conversion can be applied for high-order harmonic generations. The important laser parameters include the laser wavelength, spectral width, pulse duration, pulse energy, pulse repetition rate, and beam quality. It is difficult, however, to achieve in a single step all the right parameters from a diode-pumped solid-state laser. Diode-pumped solid-state lasers operated at kilohertz and millijole level are basically Nd-doped lasers pumped with diode-laser sources near 800 mn. When they are simply Q-switched, these lasers produce laser pulses of 30 to 100 ns in duration. With the nonlinear crystals available nowadays, this pulse duration is too long to produce an effective deep UV generation without optical damaging. The beam quality and the pulse energy are competitive parameters of these lasers. Because of their simplicity and high pulse energy, these lasers are popular and less expensive. They are commercial available from Cutting Edge Optronics of Saint Charles, Mo., and Lee Laser of Orlando, Fla.
A more complicate master-oscillator and power-amplifier laser system can produce shorter pulses and better beam quality at a much higher cost. Such a system may have a Q-switched oscillator pumped longitudinally by a high brightness diode laser. The oscillator produces a low-energy pulse of about 10 ns in duration. A diode-pumped power amplifier boosts the pulse energy to the millijole level. Deep UV generation with these lasers can achieve an efficiency of about 5%. With these 10-ns laser sources, optical damage has been an important issue on the nonlinear crystal for deep UV generation near 200 nm. Continuous movement of the nonlinear crystal may be required to avoid optical damage (U.S. Pat. No. 5,825,562 to Lai et al.) Laser systems of master-oscillator and power-amplifier are commercially available from Lambda Physik GmbH of Germany and from Continuum of Santa Clara, Calif.
An even more complicate master-oscillator and power-amplifier laser system may include a mode-locked oscillator and a regenerative amplifier. Pulses of about 100 ps in duration can be generated and pulse energy of about 1 millijole can be obtained at kilohertz repetition rate. A conversion efficiency of about 10% can be achieved for deep TV generation. Optical damage on the UV crystal is not much a problem with these short-pulse systems. The operation condition and maintenance of these systems are, however, so demanded that they be mainly used in laboratory environment. Commercial product is available from Spectra-Physics Lasers of Mountain View, Calif.. System price can be well over $200,000.
Another limitation for diode-pumped solid-state lasers operated at kilohertz and millijole level is the laser wavelength. Only for the Nd-doped laser materials, the diode pump sources have been well developed to have enough power and lifetime. Practically, laser wavelength from these lasers is limited to near 1060 or 1320 nm. As a result, their second harmonics is limited to near 530 or 660 nm.